Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 281: Which molecule acts as the primary electron carrier in the Krebs cycle?

A. NAD
B. FADH₂
C. NADPH
D. NADH (Correct Answer)

Explanation: ***NADH*** - **NADH** (reduced nicotinamide adenine dinucleotide) is the **primary electron carrier** produced during the Krebs cycle. - **Three molecules of NADH** are generated per cycle (at isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase steps). - These high-energy electrons are transferred to the **electron transport chain** to generate approximately **7.5 ATP per NADH**. *NAD+* - **NAD+** (oxidized nicotinamide adenine dinucleotide) is the coenzyme that *accepts* electrons during the Krebs cycle. - It is the **oxidized form** that gets reduced to NADH, not the carrier itself. *FADH₂* - **FADH₂** (reduced flavin adenine dinucleotide) is also produced in the Krebs cycle at the **succinate dehydrogenase** step. - However, only **one molecule of FADH₂** is produced per cycle compared to three NADH molecules. - FADH₂ generates approximately **5 ATP** in the electron transport chain, making NADH the quantitatively dominant electron carrier. *NADPH* - **NADPH** (reduced nicotinamide adenine dinucleotide phosphate) is NOT involved in the Krebs cycle. - It is primarily used in **anabolic pathways** such as fatty acid synthesis, cholesterol synthesis, and the **pentose phosphate pathway**. - NADPH serves as a **reducing agent** in biosynthetic reactions and protects against oxidative stress.

Question 282: What is the total number of dehydrogenases involved in the Krebs cycle?

A. 3
B. 2
C. 4 (Correct Answer)
D. 5

Explanation: ***4*** - There are four major **dehydrogenase enzymes** that catalyze oxidation-reduction reactions in the Krebs cycle. - These enzymes are **isocitrate dehydrogenase**, **α-ketoglutarate dehydrogenase complex**, **succinate dehydrogenase**, and **malate dehydrogenase**. *3* - This count is incorrect as it omits at least one key dehydrogenase involved in the Krebs cycle's oxidative steps. - A count of three would exclude one of the enzymes responsible for generating **NADH** or **FADH2**. *2* - This number is significantly underestimated, as the Krebs cycle involves multiple steps where a substrate is oxidized and a coenzyme is reduced. - Such a low number would fail to account for the multiple points of **NADH** and **FADH2** generation. *5* - This count is incorrect, as there are specifically four well-established dehydrogenase enzymes within the Krebs cycle responsible for the production of **NADH** or **FADH2**. - No additional dehydrogenase beyond the four listed plays a primary role in the canonical Krebs cycle.

Question 283: Which coenzyme serves as the primary electron acceptor in multiple dehydrogenase reactions of the Kreb's cycle?

A. NAD (Correct Answer)
B. NADP
C. NADPH
D. NADH

Explanation: ***NAD⁺ (NAD)*** - **NAD⁺ (Nicotinamide Adenine Dinucleotide)** serves as the primary **electron acceptor** in the Kreb's cycle, being reduced to **NADH** in three key dehydrogenase reactions. - These reactions occur at: **isocitrate dehydrogenase**, **α-ketoglutarate dehydrogenase**, and **malate dehydrogenase** steps. - The **NAD⁺/NADH** coenzyme system is essential for extracting energy from acetyl-CoA, with NADH subsequently donating electrons to the **electron transport chain** for ATP synthesis. *NADP* - **NADP⁺ (Nicotinamide Adenine Dinucleotide Phosphate)** is primarily involved in **anabolic reactions**, such as **fatty acid synthesis** and the **pentose phosphate pathway**. - While structurally similar to NAD⁺ (differing only by a phosphate group), it functions in different metabolic pathways and is not utilized in the **Kreb's cycle**. *NADPH* - **NADPH** is the reduced form of **NADP⁺** and functions as a reducing agent in various **biosynthetic pathways**, including synthesis of **fatty acids**, **cholesterol**, and **nucleotides**. - It also plays a crucial role in **antioxidant defense** (glutathione reduction) and the **respiratory burst** in phagocytes. - NADPH is not involved in the **Kreb's cycle**, which uses the NAD⁺/NADH system instead. *NADH* - **NADH** is the **reduced form** of NAD⁺ produced during the Kreb's cycle reactions. - While NADH and NAD⁺ are two forms of the same coenzyme, the question asks for the **electron acceptor** form, which is **NAD⁺** (oxidized form). - **NADH** carries the extracted electrons to **Complex I** of the electron transport chain, where it is reoxidized back to NAD⁺, generating approximately **2.5 ATP** per NADH molecule.

Question 284: Which of the following metabolic processes is associated with increased Basal Metabolic Rate (BMR)?

A. Increased body fat store
B. Increased lipogenesis
C. Increased glycolysis (Correct Answer)
D. Increased glycogenesis

Explanation: ***Increased glycolysis*** - Among the given options, **increased glycolysis** is the best answer as it represents **active catabolic metabolism** that generates ATP to meet energy demands. - While glycolysis itself doesn't directly increase BMR, **increased glycolytic activity occurs in metabolically active tissues** and reflects higher cellular energy turnover. - Tissues with higher metabolic rates (muscle, brain, liver) have increased glycolysis to meet their ATP demands, making this the most appropriate choice among the options provided. *Increased body fat store* - **Adipose tissue** is metabolically **less active** than lean tissue (muscle, organs). - Increased body fat typically results in a **lower BMR per unit body weight** because fat tissue has minimal metabolic activity compared to muscle. - Greater fat stores are associated with lower, not higher, metabolic rate. *Increased lipogenesis* - **Lipogenesis** (synthesis of fatty acids and triglycerides) is an **anabolic storage process**. - This process occurs during energy surplus and represents a state of **reduced energy expenditure** relative to energy intake. - Storage processes like lipogenesis are associated with **lower overall metabolic activity**, not increased BMR. *Increased glycogenesis* - **Glycogenesis** (synthesis of glycogen from glucose) is an **anabolic storage process** occurring primarily in liver and muscle. - This represents **energy storage**, not energy expenditure, and occurs during fed states when energy demands are being met. - Storage processes do not increase BMR; they indicate surplus energy being stored for later use.

Question 285: Which of the following is not a cofactor required by pyruvate dehydrogenase?

A. CoA
B. NAD
C. FAD
D. Biotin (Correct Answer)

Explanation: ***NAD*** - **NAD+ (Nicotinamide adenine dinucleotide)** is required by the dihydrolipoyl dehydrogenase (E3) component of the pyruvate dehydrogenase complex to accept electrons and form NADH. - This cofactor is crucial for the regeneration of the oxidized lipoamide, allowing the complex to continue its catalytic cycle. *FAD* - **FAD (Flavin adenine dinucleotide)** is also a cofactor for the dihydrolipoyl dehydrogenase (E3) enzyme, accepting electrons from reduced lipoamide before transferring them to NAD+. - It is tightly bound to the E3 enzyme and undergoes reversible oxidation-reduction during the reaction. *Biotin* - **Biotin** is primarily a cofactor for **carboxylase enzymes**, such as pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate. - It is **not involved** in the pyruvate dehydrogenase complex reaction, which is an oxidative decarboxylation, not a carboxylation. *CoA* - **Coenzyme A (CoA)** is essential for the pyruvate dehydrogenase complex, as it accepts the acetyl group from pyruvate to form **acetyl-CoA**. - Acetyl-CoA is the product of the reaction and serves as the entry molecule into the **Krebs cycle**.

Question 286: Hyperammonaemia inhibits the TCA cycle by depleting which of the following?

A. succinate
B. α-ketoglutarate (Correct Answer)
C. malate
D. fumarate

Explanation: ***a keto glutarate*** - **Hyperammonemia** leads to the depletion of **α-ketoglutarate** through its amination to form **glutamate** by glutamate dehydrogenase and subsequently glutamine by glutamine synthetase. - The removal of **α-ketoglutarate** from the TCA cycle impairs its ability to produce energy and essential intermediates, contributing to neurological dysfunction in hyperammonemia. *succinate* - **Succinate** is an intermediate in the TCA cycle, but its depletion is not the primary mechanism by which hyperammonemia inhibits the cycle. - The direct consumption of **α-ketoglutarate** for ammonia detoxification is the more direct and significant impact. *malate* - **Malate** is another intermediate in the TCA cycle but is downstream from **α-ketoglutarate**. - Its depletion is a consequence of overall TCA cycle inhibition, not the initial cause mediated by hyperammonemia. *fumarate* - **Fumarate** is also a TCA cycle intermediate and is produced after succinate. - Its levels would be affected by the overall inhibition of the cycle, but it is not the direct target or substrate for ammonia detoxification that depletes the cycle.

Question 287: Which of the following is the most reactive free radical?

A. Alkyl radical
B. Superoxide radical
C. Peroxide radical
D. Hydroxyl radical (Correct Answer)

Explanation: ***Hydroxyl radical*** - The **hydroxyl radical (•OH)** is the most reactive free radical in biological systems due to its extremely high oxidation potential and short half-life. - It readily reacts with virtually all cellular macromolecules, including **DNA, proteins, and lipids**, causing widespread damage. *Peroxide radical* - The **peroxide radical (ROO•)**, or more specifically the peroxyl radical, is less reactive than the hydroxyl radical, but still significant in lipid peroxidation. - It plays a role in propagating chain reactions of **lipid damage** in cell membranes. *Alkyl radical* - **Alkyl radicals (R•)** are generally formed as intermediates during the abstraction of hydrogen atoms from saturated compounds. - While reactive, they are typically less reactive and less frequently encountered in biological systems compared to oxygen-centered radicals like the hydroxyl radical. *Superoxide radical* - The **superoxide radical (O₂•−)** is a relatively less reactive free radical compared to the hydroxyl radical, but it is the precursor to many other reactive oxygen species (ROS). - It is primarily involved in **initiation of oxidative stress** and can lead to the formation of more damaging species through reactions like the Haber-Weiss reaction.

Question 288: Which metabolic pathway is least active during 12 days of fasting?

A. Gluconeogenesis
B. Glycogenolysis (Correct Answer)
C. Ketogenesis
D. Lipolysis

Explanation: ***Correct: Glycogenolysis*** - **Glycogenolysis**, the breakdown of glycogen stores, is very active during the **initial hours of fasting** (first 24-48 hours) to maintain blood glucose levels. - However, after **12 days of fasting**, liver and muscle **glycogen stores are completely depleted**, making this pathway **essentially inactive** or the least active of all the metabolic pathways. - Once glycogen is exhausted, this pathway cannot contribute further to energy metabolism. *Incorrect: Gluconeogenesis* - This pathway becomes **increasingly active** during prolonged fasting to **synthesize new glucose** from non-carbohydrate precursors (amino acids, lactate, glycerol). - Essential for maintaining blood glucose for **glucose-dependent tissues** like red blood cells and parts of the brain that haven't fully adapted to ketones. - Remains a **crucial and active pathway** throughout prolonged fasting. *Incorrect: Ketogenesis* - **Ketogenesis** is **highly active** during prolonged fasting, producing **ketone bodies** (acetoacetate, β-hydroxybutyrate) from fatty acids in the liver. - Provides the **primary alternative fuel** for the brain (up to 70% of brain energy needs) and other tissues. - This is a **key metabolic adaptation** to preserve protein and glucose during starvation. *Incorrect: Lipolysis* - **Lipolysis** (breakdown of triglycerides into fatty acids and glycerol) is **highly active** during fasting to mobilize stored energy. - Provides **fatty acids** for direct oxidation by most tissues and **glycerol** as a gluconeogenic substrate. - A **fundamental process** for energy supply during nutrient deprivation.

Question 289: In the malate shuttle, how many ATPs are produced from one NADH?

A. 1 ATP
B. 3 ATP
C. 2 ATP
D. 2.5 ATP (Correct Answer)

Explanation: ***2.5 ATP*** - In the **malate-aspartate shuttle**, mitochondrial **NADH** is regenerated from cytosolic NADH, and then enters the electron transport chain at **Complex I**. - **Complex I** entry means that NADH contributes to the pumping of enough protons to generate approximately **2.5 ATP** through oxidative phosphorylation. *1 ATP* - **1 ATP** is not the direct equivalent produced from the reoxidation of one NADH via the malate shuttle into the electron transport chain. - This value is typically associated with the direct hydrolysis of **ATP** or the energy equivalent of **GTP** produced in the citric acid cycle. *3 ATP* - Historically, **3 ATP** was the accepted stoichiometry for one NADH, but more accurate measurements of proton pumping and ATP synthase activity have revised this. - The value of 3 ATP per NADH does not reflect the most current understanding of mitochondrial bioenergetics. *2 ATP* - **2 ATP** is the approximate yield for **FADH2** entering the electron transport chain at **Complex II**, bypassing Complex I, and thus pumping fewer protons. - This value is not applicable to NADH transferred via the malate-aspartate shuttle, as NADH enters at Complex I.

Question 290: The mechanism of action of uncouplers of oxidative phosphorylation involves:

A. Inhibition of ATP synthase
B. Stimulation of ATP synthase
C. Disruption of proton gradient across the inner membrane (Correct Answer)
D. Blocking electron transport chain complexes

Explanation: ***Disruption of proton gradient across the inner membrane*** - Uncouplers such as **2,4-dinitrophenol** increase the permeability of the **inner mitochondrial membrane** to protons. - This dissipates the **proton motive force** that is normally used by ATP synthase to produce ATP, leading to the uncoupling of electron transport from ATP synthesis. *Inhibition of ATP synthase* - Inhibitors of ATP synthase directly block the enzyme's activity, preventing the synthesis of ATP while the **proton gradient** remains intact. - This mechanism is distinct from uncouplers, which allow electron transport to continue while dissipating the proton gradient. *Stimulation of ATP synthase* - Uncouplers do not stimulate ATP synthase; rather, their action prevents ATP synthase from effectively utilizing the **proton gradient** for ATP production. - Stimulation of ATP synthase would lead to increased ATP synthesis, which is contrary to the effect of uncouplers. *Blocking electron transport chain complexes* - Inhibitors of the **electron transport chain** (e.g., cyanide, rotenone) directly prevent the flow of electrons, thereby preventing the pumping of protons and the formation of a **proton gradient**. - Uncouplers, in contrast, allow electron transport to proceed but dissipate the proton gradient after it has been established.

Practice by Chapter

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ATP as Energy Currency

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Tricarboxylic Acid Cycle

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Electron Transport Chain

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Oxidative Phosphorylation

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Mitochondrial Diseases

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Uncouplers and Inhibitors of Oxidative Phosphorylation

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Shuttle Systems: Malate-Aspartate and Glycerol-Phosphate

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Energy Yield from Nutrients

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Metabolic Rate and Basal Metabolism

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